Electrolytic method of and compositions for stripping...

Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy

Reexamination Certificate

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C205S723000, C252S500000

Reexamination Certificate

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06332970

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an electrical stripping process and compositions for stripping electroless nickel from a substrate. An electrical stripping process can be described as the reverse of electroplating. While electroplating applies a coating of metal to a substrate, an electrical stripper removes a coating from the substrate. The coating is dissolved during electrolysis by combining with negative chemical ions in a bath, which are attracted to its surface by its positive potential. While the object of a stripper is to remove a coating without damage to the underlying substrate, most anodic reactions cannot differentiate between the coating and the substrate resulting in etching of that substrate.
Electroless nickel is a very chemical resistant coating and is difficult to attack. An illustration of its chemical resistant is the “nitric acid drop test”. A drop of concentrated nitric acid is applied to the electroless nickel coating's surface. If any etching occurs within a set time period, the test fails. The discovery of relatively mild chemical formulations and procedures that will rapidly strip these very chemical resistant coatings is of great economic value. This is especially true when the stripping process can be made environmentally friendly and safe to use in the recovery of defectively plated parts.
DESCRIPTION OF THE RELATED ART
Since the invention of autocatalytic chemical nickel deposition (now commonly called electroless nickel plating), the excellent chemical resistance of the deposit has found wide use in protecting a variety of manufactured articles. This chemical resistance has been improved over the years through innovative formulations that modify the structure of the deposit to make it even more resistant to chemical attack. Not surprisingly, the task of stripping electroless nickel while saving the substrate has become increasingly difficult.
Various methods have been used to attempt to strip electroless nickel with varying degrees of success. One of the earliest methods tried was an electrolytic stripper which was in common use for stripping electrolytic nickel. The stripper was made from concentrated sulfuric acid, utilizing reverse current. Typically, a part to be stripped was immersed in a stripping bath and connected to the positive (anodic) lead of a direct current source. Current traversed the bath to the negative electrode (cathode) which was usually made of lead or graphite. The method utilized the principal of passivity, i.e., when the nickel deposit had been dissolved by the action of the sulfate ion, unlike the nickel, the chemical characteristics of the steel substrate in the presence of the very concentrated sulfuric acid would make the steel passive to attack and oxygen would be evolved. This method failed when trying to strip most electroless nickel deposits because, just as steel, the electroless nickel became passive.
U.S. Pat. No. 4,356,069 (Cunningham) describes an electrical nickel stripper composed of concentrated sulfuric acid, chromic acid, and hydrogen peroxide. This formulation claimed to strip chromium as well as nickel from ferrous substrates. While the patent does not claim that the formulation will strip electroless nickel deposits, it suffers from the disadvantages of high disposal costs of its large hazardous chromium and sulfuric acid content, and the added step of having to remove an oxide layer from the substrate before replating can be attempted.
Another chromic acid based electrical nickel stripper is described in U.S. Pat. No. 4,647,352 (Cook). This patent states that the combination of chromic acid, phosphoric acid, and sodium bisulfite can be used to electrically strip electroless nickel. The formulation suffers from its sensitivity to the introduction of sulfate ions to the bath which caused it to etch the substrate mandating the use of barium carbonate treatment. Disposal of spent stripping baths is very expensive due to the high chromic acid content.
U.S. Pat. No. 4,664,763 (McMullen et al.) discloses an aqueous stripping solution comprising chromic acid utilizing defectively plated parts as either or both the anode and cathode of an electrolytic stripping cell and applying an alternating current across the electrodes to strip the nickel coatings. Electrolytic and electrolysis nickel coatings are said to be stripped by this method. This stripping method cites several optional etch inhibitors, such as potassium iodide, to help prevent etching of the substrate. This method suffers from the possibility of etching and the high disposal cost of the high chromic acid content.
Other electrolytic stripping processes utilize nitrates, normally ammonium or sodium nitrate, at a concentration of about three pounds per gallon. These processes use reverse direct current to remove electroplated nickel and some low phosphorous electroless nickel deposits from iron and steel. As nickel dissolves, the stripping bath undergoes rapid pH changes that produce heavy sludging problems. Sporadic passivity of large areas of unstripped nickel occur as the exposed base metal becomes passive under the influence of the electric current causing an incompletely stripped part. The problem is more pronounced when trying to strip electroless nickel deposits above about seven percent phosphorous.
Immersion nickel strippers (no electrical current needed) were introduced that utilized soluble nitrobenzene compounds in solution with cyanide compounds to strip nickel deposits. These strippers found limited use for stripping electroless nickel with a low phosphorous content, i.e., 1%-7%, however, the higher phosphorous content electroless nickel deposits either did not strip or stripped at such low rates that the process proved to be impractical. With the advent of pollution controls, the use of cyanide became more and more expensive as the liability of disposal increased.
Nickel stripping baths that utilize water soluble nitrobenzene compounds, either ammonia and ammonium salts or ethylene diamine and/or its homologs, have gained wide acceptance. These baths work well on deposits of electrolytic nickel and some low phosphorous electroless nickels have achieved stripping rates of about 0.001 inch/hour, however, the high phosphorous content electroless deposits slowed the removal to, on average, less than 0.0003 inch an hour. Proper disposal of these types of stripping baths is very expensive because of their toxicity and high chelating power. These baths operate at about 160-190° Fahrenheit to strip electroless nickel coatings and are very vulnerable to damage by heat through loss of volatile chemicals components as well as thermal chemical decomposition.
U.S. Pat. No. 4,554,049 (Bastenbeck) discloses an immersion nickel stripper which uses sulfamic acid, hydrogen peroxide, nitrates, and chlorides. The patent claims to strip low phosphorous electroless nickel (less than 7%) but cannot effectively strip deposits of greater phosphorous content.
U.S. Pat. No. 4,720,332 (Coffey) describes a nickel stripping bath that utilizes soluble nitrobenzene compounds, Zwitterions (as chelating agents), sulfide producing compounds, carbonates, and the use of reverse current for the fast removal of electroless nickel deposits. While this method strips electroless nickel fast (up to 0.002 inch/hour) and works well, it sometimes microscopically etches in high current density areas dulling highly polished surfaces.
Thus, it is the objective of this invention to improve the art of stripping electroless nickel by providing formulations to electrolytically strip electroless nickel deposits of both low and high phosphorous content with equal high efficiency and ease.
It is also an objective of this invention to provide formulations to electrolytically strip electroless nickel with improved resistance to etching the substrate.
It is a further objective to provide a choice of alternative stripping formulations that are safer to use in the workplace and more environmentally friendly by eliminating chelates, nitrates, chromates, cyanide, and

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